The MIM (Metal Injection Molding) process and the inductor fabrication process require iron powders having a small particle size and a specific shape. Preferably, the size is smaller than 20 μm, and the shape is nearly spherical. The current methods of producing iron powders include millscale reduction, magnetite powder reduction, gas atomization, water atomization, electrolysis, and carbonyl decomposition. The iron powders produced using millscale reduction, magnetite powder reduction, gas atomization, and water atomization methods have a coarse mean particle size (normally greater than 20 μm). Although smaller particles can be screened from the abovementioned iron powders, only a small proportion of sufficiently small particles can be obtained from the abovementioned iron powder. Therefore, these methods are costly. The electrolysis method can produce iron powders having a smaller particle size. However, these particles have a dendritic shape. Thus, this iron powder has a low packing density and poor flowability. In contrast, the carbonyl decomposition method can fabricate carbonyl iron powders featuring a small particle size (about 2-10 μm), high packing density, spherical shape, high purity, and superior sinterability. Furthermore, the carbonyl decomposition method can be used to mass-produce iron powders. Therefore, the carbonyl decomposition method is usually used for fabricating iron powders for the MIM process and the inductor fabrication process.
As for the conventional carbonyl decomposition method, U.S. Pat. No. 4,652,305 and No. US2011/0162484 disclosed an iron powder fabrication method, which comprises a high-pressure synthesis process and a thermal decomposition process. Firstly, sponge iron powders or reduced iron powders are reacted with carbon monoxide under a high pressure to form gaseous iron pentacarbonyl (Fe(CO)5). Next, the pressure and temperature are decreased, and the gaseous iron pentacarbonyl becomes liquid iron pentacarbonyl. Then the iron pentacarbonyl is gasified and thermally decomposed to carbonyl iron powders.
The advantages of the carbonyl iron powder include spherical shape, high packing density, small particle size, superior sinterability, and capability of mass production. However, the fabrication process thereof is complicated and has safety concerns due to the colorless, odorless, and toxic carbon monoxide involved. The fabrication of carbonyl iron powder also requires high temperatures and high-pressure equipment, airtight thermal decomposition devices, and safety-protection facilities. For these reasons, the capital investment and technical challenges of fabricating carbonyl iron powders are very high for the user. Since the user commonly lacks facilities to fabricate the powder, the user has to pay a very high price to purchase it on the market.